youtube-dl is a small command-line program to download videos
from YouTube.com and a few more sites. It requires the Python
interpreter, version 2.6, 2.7, or 3.2+, and it is not platform
specific. It should work on your Unix box, on Windows or on
Mac OS X. It is released to the public domain, which means you
can modify it, redistribute it or use it however you like.

Optional dependency: python3
